1. Field of the Invention
This invention concerns a method of preparing a cellulose carbamate spinning solution by dissolving cellulose carbamate in sodium hydroxide solution, where the solution is prepared by a controlled two-step process.
2. Summary of the Related Art
Cellulose carbamate solutions are needed to prepare regenerated cellulose fibers and other structures. Regenerated cellulose fibers are produced in several steps. The first step consists of reacting cellulose with urea at elevated temperatures to form cellulose carbamate. The cellulose carbamate prepared in this way is then dissolved in sodium hydroxide solution and the resulting solution spun. The cellulose carbamate that is in the form of fibers or other structures can be broken down to cellulose by another process step.
Preparation of the cellulose carbamate solution plays an important role in the process described above. It is known from the literature that cellulose carbamate solutions can coagulate relatively easily and quickly and are more sensitive in this regard than viscose and must be taken into account in designing the spinning operation. The cellulose carbamate solution""s quality and stability, which are important for the processing operation and for the quality of the fibers and other structures, depend on the quality and the average degree of polymerization (DP) of the cellulose carbamate and the conditions of preparation of the solution (e.g., the sodium hydroxide concentration, the dissolving temperature, the shearing forces in effect in dissolving, plus any zincate or urea additives in the solution).
Thus, for example, it is known from German Patent No. 3,343,156 that the dissolubility can be influenced by adding urea as a solubilizer. Zincate is another solubilizer (O. T. Turunen et al., Lenzinger Berichte 59, 111-177 (1985)). The DP of cellulose carbamate may be adjusted by exposure to high energy radiation prior to or during dissolution of, for example, 5.4 wt. % cellulose carbamate in 10 or 20 wt. % sodium hydroxide at xe2x88x925xc2x0 C. (German Patent Application 34 39 714). It is also known that the weight ratio of cellulose carbamate to sodium hydroxide should usually be selected to be  less than 1 for the cellulose carbamate solution, for example, 6.8 wt. % cellulose carbamate in 9 wt. % sodium hydroxide, wherein the sodium hydroxide may be recycled from the further spinning process and its concentration adjusted prior to the dissolving step by addition or evaporation of water (German Patent Applications 35 34 357 and 35 34 371 corresponding to U.S. Pat. Nos. 4,639,514 and 4,639,515).
To make fiber production economical, however, it is necessary to process spinning solutions having the highest possible cellulose carbamate content and the lowest possible sodium hydroxide content, as is conventional for the viscose process. At the same time, the undissolved particle content in these solutions should be minimal. However, better dissolution of the cellulose carbamate and better solution quality obtained by employing a high sodium hydroxide concentration and/or additional additives to the dissolving bath are not consistent with these economic requirements.
The present invention provides a method of preparing cellulose carbamate solutions that enables preparation of acceptable cellulose carbamate spinning solutions without any other additives, and the ratio of cellulose carbamate concentration to sodium hydroxide concentration is  greater than 1. The cellulose carbamate solutions of the invention do not have any premature coagulation either.
The process comprises dissolving cellulose carbamate in an sodium hydroxide solution at temperatures below about 10xc2x0 C. following by dilution of the resulting solution with additional sodium hydroxide solution and/or water. It has been unexpectedly found that this process obviates the aforementioned disadvantages inherent in the prior art processes.
The foregoing merely summarizes the present invention and is not intended nor should it be construed as limiting the invention in any manner.
The present invention provides for the first time a two-step process for preparing a cellulose carbamate solution. In the first step, an 8-15% cellulose carbamate solution is prepared with a concentration of sodium hydroxide solution of about 7-12% (which is higher than in the second step). It is preferable for the cellulose carbamate concentration to be in the range of 9-12% and the sodium hydroxide concentration to be in the range of 8-10% (all % by weight).
Subsequently, this solution is diluted by adding a dilute sodium hydroxide solution and/or water so that the final sodium hydroxide concentration in the cellulose carbamate solution is less than 9% (preferably xe2x89xa68%). The cellulose carbamate concentration may be in the range of approximately 6-9%. The weight ratio of cellulose carbamate to alkali should be  greater than 1 in both steps, i.e., the concentration of the cellulose carbamate solution is higher than that of the sodium hydroxide solution.
More specifically, the invention provides a process for preparing a cellulose carbamate solution, the process comprising
a) preparing an 8% to 15% cellulose carbamate solution with a sodium hydroxide concentration of 7% to 12% by dissolving cellulose carbamate in a sodium hydroxide solution at a temperature of  less than 10xc2x0 C., and
b) adjusting the sodium hydroxide concentration of the cellulose carbamate solution to  less than 9% by adding to the cellulose carbamate solution either
i) a single diluent comprising a dilute sodium hydroxide solution or water, or
ii) a first diluent followed by a second diluent, wherein if the first diluent is one of water or a dilute sodium hydroxide solution and the second diluent is the other of water or a dilute sodium hydroxide solution,
provided that the weight ratio of cellulose carbamate to sodium hydroxide in both (a) and (b) is xe2x89xa71.
A cellulose carbamate solution with such a high cellulose carbamate content relative to the sodium hydroxide content at a sodium hydroxide concentration of  less than 9% was previously unknown. It should be noted in particular that the improved dissolubility with the present method is independent of the DP of the cellulose carbamate used. The two-step process yields much better results in all cases with a cuprammonium DP of 200-400 as compared to the one-step prior art method (see Comparative Example).
Dilution from the first step to the second is performed with a sodium hydroxide solution diluent with a concentration of at least 2 percentage points less than in the first step, preferably with a concentration sodium hydroxide of xe2x89xa64%. As an alternative to this, it is also possible to initially use a diluted sodium hydroxide solution with a concentration of about 2 percentage points less than in the first step as the diluent and then to add water until reaching the desired final concentration. With the present method it is also advantageous to use vigorous stirring during the two-step dissolving process. Preferably, the forced circulation and the shearing forces should be as high as possible, but at the same time the heat development as low as possible. It has also proven advantageous if the diluent is added more slowly at the start of dilution in relation to the end point of dilution. Thus, in this case, the rate of addition of diluent is increased until ultimately the sodium hydroxide solution can be replaced by water. When the diluent is added very slowly and mixed thoroughly, it is also possible to use water as the diluent from the very beginning. The diluent can be added more quickly the higher its sodium hydroxide concentration is. Preferably the diluent is added in such a way that at no time does the dissolved cellulose carbamate precipitate.
Instead of pure solutions of sodium hydroxide in water, technical or recycled sodium hydroxide solutions or sodium hydroxide solutions containing additives like urea or zincate may be used.
In both steps, the dissolving process is carried out at temperatures of  less than 10xc2x0 C., preferably  less than 5xc2x0 C.
It has surprisingly been found that the cellulose carbamate solution does not coagulate in the present method (see also Table 1).